Book/Dissertation / PhD Thesis

FZJ-2017-07410

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Characterization of effective hydraulic properties of unsaturated porous media using spectral induced polarization (SIP)

Breede, K. (Corresponding author)FZJ*

2013
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-875-4

Please use a persistent id in citations: http://hdl.handle.net/2128/15817

Abstract: Groundwater is a life-sustaining but vulnerable resource which is endangered by contaminants. Soil acts as an important protective buffer for groundwater and, therefore, the understanding of flow and transport processes in soils is of utmost importance. However, the prediction capabilities of flow and transport models in the vadose zone are often limited due to an insufficient knowledge about the structural and textural heterogeneity of the soil. To obtain more information about soil structure, texture and heterogeneity, as well as hydraulic parameters, non-invasive electrical methods may be employed in laboratory and field-scale studies. One of the more promising electrical methods is spectral induced polarization (SIP), which measures the complex electrical conductivity in the low-frequency range from 1 mHz to 45 kHz. Recently, this method has been used to predict the saturated hydraulic conductivity of consolidated and unconsolidated porous media. To better understand the mechanisms causing polarization and to extend the range of SIP applications to the vadose zone, it is important to investigate how the SIP response is affected by water content. In addition, it might be possible to relate the SIP response to the unsaturated hydraulic conductivity. Therefore, the general aim of this thesis is the determination of effective hydraulic properties of unsaturated and unconsolidated porous media from SIP measurements. In a first step, a laboratory measurement setup was developed that allows combined electrical and hydraulic measurements on unconsolidated porous media. The experimental design allows draining samples in various consecutive pressure steps and water outflow is automatically recorded to determine the water content. Measurements of the complex electrical conductivity are conducted using a high-accuracy electrical impedance spectrometer. Experiments were conducted on unconsolidated quartz sand and three sand-clay mixtures with 5, 10, and 20 weight-percent clay. The measured complex electrical conductivity was interpreted using a Debye decomposition approach that provides the DC resistivity, the total chargeability, and a distribution of relaxation times. The influence of water content on electrical properties like real and imaginary part of the complex electrical conductivity, phase shift, and the parameters obtained from Debye decomposition was investigated for all four artificially mixed sediment samples. The measured resistivity magnitude and phase spectra and their dependence on water content are clearly different for each mixture. For pure sand, the phase values increased with decreasing water content over the entire frequency range and a phase peak appeared for moderate to low water content. The phase spectra of the sand-clay mixtures show the same behavior as the pure sand. In addition, a shift of the phase peak to higher frequencies with decreasing water saturation was observed for all samples. This shift suggests that relaxation time and length become smaller with decreasing water content, which is related to the smaller pores that are still saturated at lower water content.The relationship between unsaturated hydraulic properties and SIP parameters was also investigated. The results show a clear power-law relationship between the matric potential and the peak relaxation time of the sand-clay mixtures. However, two different slopes were observed for this relationship, one for matric potentials greater than -120 cm and another one for smaller ones. The observed slope was quadratic for matric potentials > -120 cm and linear for matric potentials below -120 cm. The quadratic relationship was attributed to diffusion processes, but the linear relationship indicates a hitherto unknown relaxation process. Using the empirical relationship between the relaxation time and the matric potential, respectively pore radius, and a simplified version of the Mualem-van Genuchten model, a relationship between the relaxation time and the unsaturated hydraulic conductivity was obtained. The slope of this power-law dependence between unsaturated hydraulic conductivity and relaxation time depends on the width of the pore size distribution as expressed by the semiempirical n parameter of the Mualem-van Genuchten model. It was concluded that the unsaturated hydraulic conductivity can be determined from spectral induced polarization using the relaxation time and additional information like the n parameter of the Mualem-van Genuchten model.

Note: Dissertation, Universität Bonn, 2012

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Contributing Institute(s):

1. Agrosphäre (IBG-3)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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Database coverage:

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